Anomalous Underscreening in Electrolytes and Ionic Liquids

The way electrolytes and ionic liquids screen electric fields is crucial for many applications and studies across several fields. In particular, it is key for capacitive technologies like energy storage and energy conversion devices. During the last years, experiments discovered an unexpected long decay length in concentrated electrolytes that can be orders of magnitude longer than the Debye screening length [1,2]. A variety of theoretical approaches has been applied to explain this finding, including liquid state theories, classical density functional theory, and full-atom molecular dynamics simulations, but they all could not find and explain the anomalous underscreening. However, in a recent study we applied molecular dynamics and Monte Carlo simulations to less concentrated systems and found identical behaviour like the observed underscreening [3]. Remarkably, we could obtain the same results from a direct measurement of the charge correlations as well as from an analysis of clusters of particles. The latter led us to a description of underscreening by a minimal theory of ion pairing that, somehow, even works in concentrated electrolytes. In my presentation I will discuss the discovery and controversial discussion about underscreening, present our findings from simulations and analysis of our data, and demonstrate how a minimal theory can be deduced. Further, I will present strong reasons, why observing anomalous underscreening in dense electrolytes directly from simulations might be more demanding than expected, if not even impossible.

[1] M. A. Gebbie, M. Valtiner, X. Banquy, and J. N. Israelachvili. Ionic liquids behave as dilute electrolyte solutions. Proceedings of the National Academy of Sciences 110:9674, 2013; M. A. Gebbie, H. A. Dobbs, M. Valtiner, and J. N. Israelachvili. Long-range electrostatic screening in ionic liquids. Proceedings of the National Academy of Sciences 112:7432, 2015.

[2] A. M. Smith, A. A. Lee, and S. Perkin. The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration. The Journal of Physical Chemistry Letters 7:2157, 2016.

[3] A. Härtel, M. Bültmann, and F. Coupette. Anomalous Underscreening in the Restricted Primitive Model. Physical Review Letters 130:108202, 2023.